Knowledge of polymer microstructure is critical to understanding the potential utility of a polymer blend. Analysis of polymer microstructure has typically relied on analytical techniques capable of providing data on the chemical composition, molecular weight, and molecular weight distribution (MW/MWD) of a polymer sample.
Temperature rising elution fractionation (TREF) separates polymeric molecules based on their crystallizability. TREF separation is a two-step process in which a dissolved polymer sample is deposited onto a column filled with inert packing material by programmed cooling of the column. The sample is then redissolved into the flowing solvent or mobile phase by raising the temperature of the column slowly while flushing the column with solvent. The temperature at which the polymer fractions elute off the column is primarily a function of the extent of short-chain branching (SCB) within the sample, molecular weights, and the thermal history the polymer has experienced. TREF analysis can be carried out on two scales depending on the amount of sample to be fractionated. Typically, a polymer sample is analyzed using preparation scale or pTREF in order to generate enough sample after fractionation to carry out additional characterization of polymer microstructure. Analytical scale TREF or aTREF is considered an improvement over pTREF because the technique requires less polymer sample and the amount of eluting polymer sample can be monitored using an on-line detector. However, aTREF analysis alone provides limited information on polymer microstructure. One limitation of aTREF analysis is that aTREF does not differentiate between polymers possessing similar melting points/elution temperatures, yet dramatically different molecular weights, molecular weight distributions, SCB distribution across the molecular weight distribution and long-chain branching distribution across the molecular weight distribution. Furthermore, information obtained from pTREF is not necessarily applicable to aTREF because there are differences in thermal histories experienced by the polymer in the two processes.
Gel permeation chromatography (GPC), also known as size-exclusion chromatography (SEC), is a useful technique for characterizing the molecular weight or molecular weight distribution (MW/MWD) of a polymer sample. Separation is accomplished by injecting the sample into a column packed with a porous packing material such as a crosslinked poly(styrene-co-divinylbenzene) gel. Without wishing to be limited by theory, GPC separation is based on differences in hydrodynamic volume. Molecules too large to enter the small pores in the packing material elute from the column first while those that can enter the small pores require a longer time or larger elution volume to elute from the column.
Routine characterization of polymer microstructure requires information from both TREF and GPC analysis. A major impediment to utilizing both TREF and GPC for polymer microstructure characterization is the need to carry out offline analysis of polymer sample fractions isolated from TREF prior to subjecting the polymer sample fractions to GPC analysis. This offline processing of the polymer sample first by pTREF fractionation, collecting the fractionated samples and then by GPC is a very tedious one. To finish a complete pTREF run with offline GPC analysis for a polymer sample usually requires two to three months, depending on the number of individual thermal cutoffs analyzed. A need therefore exists for a quick and reliable two-dimensional technique that concurrently determines the chemical composition and MW/MWD for a given temperature slice.